Conventionally, Four-ball tester includes a ballpot with three steel balls of ½ inch diameter positioned inside the ballpot. The steel balls are held very tightly together by a conical ring that is secured with a lock nut to hold balls in place. A test lubricant is filled in the ballpot submerging the three balls. The four-ball tester includes a heater block to heat the test lubricant to a predetermined test temperature. Further, a bearing supports the ballpot from the bottom. Three balls in the ballpot are forced against a fourth ball held in a collet attached to a spindle which is in-turn connected to an electric motor. The force between the lower three balls and the upper fourth ball may be measured using a load cell to ensure that load is within the specified range depending on test procedure. Upon rotation of the top ball, a force is transmitted to the test balls in the ballpot, tending to rotate the ballpot. A torque sensor is operably connected to the ballpot which prevents its rotation due to friction torque, thereby measuring frictional torque as reaction.
In wear preventive test, the fourth top ball is rotated against the three balls in the ballpot for a predetermined time period. At the end of the test the motor is stopped, the load is removed, and the ballpot is taken off the tester. The lubricant in the ballpot is drained. The ballpot with the three balls still locked in place is kept under a microscope to measure the size of wear scars. The wear scars result from the top ball rotating against each of the bottom three balls under the test load. The wear scars are measured for each of the bottom three balls. In general, each of the three bottom balls will have a wear scar that is very similar in size and shape. A measurement is made with a microscope of each wear scar diameter, along the major and minor diameters. A total of six measurements are taken, two for each ball, and then the average of the six readings is considered to be the wear scar diameter for a given test fluid under a specific test method.
For extreme pressure test, the fourth top ball is rotated against the three balls in the ballpot for a predetermined time by constantly increasing the load in predetermined steps. At the end of the test, the balls weld to each other and the motor ceases. Once welding has occurred the load is removed, and the balls are taken out of the tester. The load before the weld and at the point of weld are analyzed to measure the extreme pressure test parameters.
However, existing four-ball tester has numerous drawbacks. The main drawback in the conventional machines is that each four ball tester is manufactured to perform only one type of specific test. For example, when a wear preventive test is conducted, the tester is operated for 392 N while for an extreme pressure test, the tester is operated up to 800 kgf (7846 N). Further, no means is provided to directly measure the exact actual load on the test balls. The load cell for wear preventive test and extreme pressure test will have accuracy not more than 0.1%. Therefore to meet the requirements of ASTM, two different testers are manufactured. This leads to additional expenses in manufacturing and maintaining.
Thus, there exists a need for a four ball tester having a high sensitivity needed to apply a test load of 392 N with variation not more than 2 N, and capable of applying test loads up to 800 kgf. Thus eliminating the need for two different machines to conduct wear preventive test and extreme pressure test so as to meet requirements of ASTM D2266, ASTM D 2783, ASTM D 2596, ASTM D 5183, ASTM D 4172, and ASTM D 3702.